Added stackdriver plugin.

Please let me know if i should change something. I tried my best. I tested it by pushing some riemann-health related metrics to stackdriver as well by creating a trial account.
@aphyr I referred to this documentation: http://support.stackdriver.com/customer/portal/articles/1491766-sending-custom-application-metrics-to-the-stackdriver-system
Here also they are setting timestamp key to the current timestamp. The above documentation says like this: "The gateway message includes the timestamp the message is created or submitted."